Journal of Marine Science and Engineering Review Role of Upper-Flow-Regime Bedforms Emplaced by Sediment Gravity Flows in the Evolution of Deltas Svetlana Kostic 1,*, Daniele Casalbore 2, Francesco Chiocci 2, Jörg Lang 3 and Jutta Winsemann 3 1 Computation Science Research Center, San Diego State University, San Diego, CA 92182, USA 2 Sapienza University of Rome, 00185 Rome, Italy; [email protected] (D.C.); [email protected] (F.C.) 3 Institute for Geology, Leibniz University Hannover, 30167 Hannover, Germany; [email protected] (J.L.); [email protected] (J.W.) * Correspondence: [email protected] Received: 20 November 2018; Accepted: 27 December 2018; Published: 4 January 2019 Abstract: Upper-flow-regime bedforms and their role in the evolution of marine and lacustrine deltas are not well understood. Wave-like undulations on delta foresets are by far the most commonly reported bedforms on deltas and it will take time before many of these features get identified as upper-flow-regime bedforms. This study aims at: (1) Providing a summary of our knowledge to date on deltaic bedforms emplaced by sediment gravity flows; (2) illustrating that these features are most likely transitional upper-flow-regime bedforms; and (3) using field case studies of two markedly different deltas in order to examine their role in the evolution of deltas. The study combines numerical analysis with digital elevation models, outcrop, borehole, and high-resolution seismic data. The Mazzarrà river delta in the Gulf of Patti, Italy, is selected to show that upper-flow-regime bedforms in gullies can be linked to the onset, growth, and evolution of marine deltas via processes of gully initiation, filling, and maintenance. Ice-marginal lacustrine deltas in Germany are selected as they illustrate the importance of unconfined upper-flow-regime bedforms in the onset and evolution of distinct delta morphologies under different lake-level trends. Keywords: marine and lacustrine deltas; transitional upper-flow-regime bedforms; cyclic steps; antidunes; delta evolution; confined or unconfined setting; foreset-bottomset transition; gullies 1. Introduction Deltas can be classified based on a variety of criteria. The most conventional, process-based classification defines delta types depending on the relative contribution of fluvial, wave, or tidal energy flux that was dominant during deposition at the seaward edge of the delta [1–3]. Other classifications of deltas emphasize foreset/topset geometry [4,5], sediment grain size [2], or delivery system type [6–8]. More recent studies demonstrate that external geometry and internal characteristics of deltas greatly vary in response to the falling and rising of sea level [9–13]. This paper focuses on upper-flow-regime bedforms observed on diverse deltas in marine and lacustrine settings at water depths of up to about 150–200 m. Parts of deltas that extend beyond water depths of 200 m are not investigated herein. Upper-flow-regime bedforms on deltas may have a crucial role in their onset, growth, and evolution. Yet, they are far less documented and understood than their deep-sea counterparts. Upper-flow-regime bedforms on deep-sea fans have received a growing recognition in the last decade or so. They have been documented numerically (e.g., References [14–16]), experimentally (e.g., Reference [17]), and in the field (e.g., References [18–23]). When it comes to deltas, the most commonly reported bedforms are undulated sediment features on delta foresets (e.g., References [24–26]). The term that is frequently used to describe them is sediment J. Mar. Sci. Eng. 2019, 7, 5; doi:10.3390/jmse7010005 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 5 2 of 29 J. Mar. Sci. Eng. 2019, 7 FOR PEER REVIEW 2 waves or sediment undulations. Sediment waves on the Mediterranean deltas, which are among the most extensivelyWhen it studiedcomes to deltas, deltas, havethe most been commonly reported forreported the last bedforms thirty years are undulated [26–43]. Earlysediment on, they features on delta foresets (e.g., References [24–26]). The term that is frequently used to describe them were interpreted as sediment deformation and slope failure features [27–30,32–35]. More recently, is sediment waves or sediment undulations. Sediment waves on the Mediterranean deltas, which are theseamong interpretations the most extensively were seriously studied questioned deltas, have been and itreported was proposed for the last that thirty many years fields [26–43]. of Early sediment waveson, were they in were fact interpreted formed by as sediment sediment transport deformatio processesn and slope [26 ,failure38,39]. features Mixed theories[27–30,32–35]. suggesting, More i.e., initialrecently, sediment these deformation interpretations and were growth seriously by differential questioned sedimentand it was accumulation proposed that patternsmany fields were of also proposedsediment (e.g., waves Reference were [in37 ]).fact formed by sediment transport processes [26,38,39]. Mixed theories Thesuggesting, interpretation i.e., initial of sediment deltaic deformation sediment wavesand growth has potentiallyby differential significant sediment accumulation implications for offshorepatterns and were coastal also proposed management, (e.g., Reference especially [37]). along heavily populated coastlines worldwide (e.g., ReferencesThe interpretation[26,44]). The of deltaic scientific sediment community waves has is potentially still debating significant on a genetic implications mechanism for offshore to these and coastal management, especially along heavily populated coastlines worldwide (e.g., References features and it will take time before many of them get identified as upper-flow-regime bedforms [26,44]). The scientific community is still debating on a genetic mechanism to these features and it emplaced by sediment gravity flows (e.g., References [15,16]). will take time before many of them get identified as upper-flow-regime bedforms emplaced by Recentsediment research gravity flows on the (e.g., Squamish References delta [15,16]). [45 –47], Mazzarrà delta [48], and (glacio) lacustrine deltas (i.e.,Recent References research [ 49on– the51]) Squamish has significantly delta [45–47], improved Mazzarrà our delta understanding [48], and (glacio) of lacustrine upper-flow-regime deltas bedforms(i.e., References on deltas. [49–51]) The aim has of thissignificantly paper is im to:proved (a) Provide our understanding a summary ofof ourupper-flow-regime knowledge to date; (b) demonstratebedforms on thatdeltas. these The aim features of this in paper both is confined to: (a) Provide and unconfineda summary of environments our knowledge areto date; most (b) likely transitionaldemonstrate upper-flow-regime that these features bedforms; in both andconfin (c)ed use and field unconfined case studies environments of the Mazzarr are mostà delta likely in Italy and glaciolacustrinetransitional upper-flow-regime deltas in Northern bedforms; Germany and (c) use in field order case to studies examine of the their Mazzarrà role indelta the inevolution Italy of deltas.and glaciolacustrine deltas in Northern Germany in order to examine their role in the evolution of deltas. Terminology used to describe deltaic deposits in the literature can be very confusing. Terminology used to describe deltaic deposits in the literature can be very confusing. Thus, an Thus, an effort was made herein to adopt terminology that is clear and generally applicable (Figure1). effort was made herein to adopt terminology that is clear and generally applicable (Figure 1). For For fine-grainedfine-grained deltas,deltas, the term term delta delta plain plain is isused used co concurrentlyncurrently with with the theterm term topset topset deposit, deposit, whereas whereas the prodeltathe prodelta slope slope incorporates incorporates foreset foreset and and bottomset bottomset deposits. deposits. For For coarse-grained coarse-grained deltas, deltas, the terms the terms deltadelta plain plain and and delta delta slope slope are are used used concurrently concurrently withwith topset and and foreset foreset deposit, deposit, respectively. respectively. FigureFigure 1. Terminology 1. Terminology for for fine-grained fine-grained and and coarse-grained coarse-grained deltas: deltas: (a () a3-D) 3-D view view of the of theMazzarrà Mazzarr riverà river delta,delta, Italy. Italy. The The location location of theof the delta delta plain/ plain/ liplip is only tentative tentative as as the the 3-D 3-D coverage coverage does does not extend not extend to to the delta plain. The flow was from right to left. (b) Gilbert-type delta reproduced experimentally at St. Anthony Falls Laboratory, USA (Experiment 2 in Kostic and Parker [43]). The flow was from left to right. J. Mar. Sci. Eng. 2019, 7, 5 3 of 29 J. Mar. Sci. Eng. 2019, 7 FOR PEER REVIEW 3 2. Summarythe of delta Knowledge plain. The flow to Date was from right to left. (b) Gilbert-type delta reproduced experimentally at St. Anthony Falls Laboratory, USA (Experiment 2 in Kostic and Parker [43]). The flow was from left 2.1. What Wasto right. Learned from Studies on Mediterranean Deltas? Urgeles2. Summary et al. of [26 Knowledge] offer a very to Date detailed account of the relevant multidisciplinary investigations of the Mediterranean deltas (Figure2). They combine previously published data with newly acquired2.1. high-resolution What Was Learned data from toStudies scrutinize on Mediterranean the origin Deltas? of the Mediterranean deltaic sediment